A "minimal" sodium channel construct consisting of ligated S5-P-S6 segments forms a toxin-activatable ionophore

The large size (six membrane-spanning repeats in each of four domains) and asymmetric architecture of the voltage-dependent Na+ channel has hindered determination of its structure. With the goal of determining the minimum structure of the Na+ channel permeation pathway, we created two stable cell lines expressing the voltage-dependent rat skeletal muscle Na+ channel (mu1) with a polyhistidine tag on the C terminus (muHis) and pore-only mu1 (muPore) channels with S1-S4 in all domains removed. Both constructs were recognized by a Na+ channel-specific antibody on a Western blot. muHis channels exhibited the same functional properties as wildtype mu1. In contrast, muPore channels did not conduct Na+ currents nor did they bind [H-3]saxitoxin. Veratridine caused 40 and 54% cell death in muHis- and muPore-expressing cells, respectively. However, veratridine-induced cell death could only be blocked by tetrodotoxin in cells expressing muHis, but not muPore. Furthermore, using a fluorescent Na+ indicator, we measured changes in intracellular Na+ induced by veratridine and a brevotoxin analogue, pumiliotoxin. When calibrated to the maximum signal after addition of gramicidin, the maximal percent increases in fluorescence (DeltaF) were 35 and 31% in cells expressing muHis and muPore, respectively. Moreover, in the presence of 1 mum tetrodotoxin, DeltaF decreased significantly to 10% in muHis- but not in muPore-expressing cells (43%). In conclusion, S5-P-S6 segments of mu1 channels form a toxin-activable ionophore but do not reconstitute the Na+ channel permeation pathway with full fidelity.